The present invention relates generally to full duplex radio frequency (RF) circuits and more particularly to reducing or eliminating transmitter signal leakage into the receive path of an RF circuit.
The accelerating demand for wireless communications such as cellular and cordless telephones, has motivated new research in RF integrated circuits (ICs). This new research emphasizes highly integrated designs, for lower cost, lower power consumption, and smaller form factors. One aspect involves locating the RF transmitter and RF receiver circuit closer and closer together with a goal of having them both on a single Integrated Circuit (IC) chip.
However, moving the RF transmitter circuit closer to the RE receiver circuit in the full duplex communication case, results in signal leakage from the transmitter circuit into the receive circuit path. This is solved conventionally by physical or mechanical isolation. Current solutions for RF communications systems having RF IC""s include either using separate chips for the transmit and receive paths or using a special isolation material to minimize the transmit signal leakage between the transmit path and the receive path. A drawback of such conventional solutions is that they limit the amount by which the size of the transmitter and receiver circuits may be reduced. Such solutions are therefore impractical for building a single chip solution that meets the tight specifications of today""s wireless standards.
An alternative to mechanical or physical isolation is to provide some sort of electronic solution. However, while techniques for cancellation of interference from external transmitter sources exist, techniques for canceling internal transmitter signal leakage in the receive path are lacking, especially if the transmit and receive circuits are on a single IC. An example of a conventional technique is given in U.S. Pat. No. 5,974,301 by Palmer, et. al, entitled xe2x80x9cFrequency Canceling System and Method,xe2x80x9d issued on Oct. 26, 1999 (hereinafter referred to as Palmer). Palmer discloses generating a reference signal and then adjusting it to be 180 degrees out of phase and equal in amplitude to an interference signal. The reference signal is then added to the interference signal to cancel out the interference signal. In Palmer, the reference signal is generated when the other external sources, called radio frequency tag devices, are not sending communication signals. Hence the reference signal is a measure of the external interference. Interference from signals outside the RF communications system has some substantial differences from internal transmitter signal leakage. Some of the differences include: the RF communications system has little or no control over these external interference sources; these techniques still rely on physical or mechanical isolation to reduce the internal transmitter leakage; and the analog RF transmitter and receiver circuitry are not on the same IC chip. Hence, there is still the problem of canceling internal transmitter leakage.
Therefore, for an RF communications system having co-located receiver and transmitter circuits, and operating in full duplex mode, there is a need for techniques that reduce or eliminate the transmitter signal leakage into the receiver path. In addition, there is a need for techniques which allow the placement of the transmitter and receiver RF circuits on a single IC.
The present invention provides a method and apparatus for reducing or eliminating the transmitter signal leakage, i.e., transmitter noise, into the receiver path of an RF communications system operating in full duplex mode. In an embodiment of the present invention a noise cancellation loop produces an estimated transmitter signal leakage and cancels it from the receive path to produce a received signal with little or no transmitter noise, Some of the advantages are that there is significant improvement in the isolation between the transmit/receive circuits, the size of the RF communications circuitry may be reduced, and the RF transmit module along with the RF receive module may be incorporated into a single RF IC chip.
In one embodiment of the present invention, a method in a RF communications system for improving a received communications signal during full duplex operation is provided. The RF communications system includes a transmitter circuit and a receiver circuit. The method determines a first estimation of a signal leakage from the transmitter circuit, when the received communications signal is not being received by the receiver circuit. Next, when the RF communications system is in full duplex operation and the transmitter circuit is generating a transmit communication signal, a second estimation of said signal leakage is determined based on the first estimation and said transmit signal. Lastly, the second estimation is combined with the received communications signal, such that the signal leakage is substantially cancelled in the received communications signal.
In another embodiment of the present invention, a method in an RF IC communications system is provided for substantially canceling transmitter noise during full duplex operation. The RF IC communications system includes a transmitter and a receiver. The RF IC communications system is calibrated by sending a calibration signal to a transmit noise cancellation loop circuit. Next loop parameters for the transmit noise cancellation loop circuit are determined. Then when in full duplex operation, a transmit noise signal based on the loop parameters is estimated, and the transmit noise signal is substantially cancelled from a received signal. In one embodiment the transmitter and receiver are on one RF IC chip. In another embodiment they are co-located, but on separate IC chips.
An RF communications system that substantially cancels transmitter noise during full duplex operation is provided in an embodiment of the present invention. The system includes: a transmit path coupled to a noise cancellation loop, and when the RF communications system is in full duplex mode, the transmit path is coupled to an antenna. Also included is a receive path coupled to the noise cancellation loop, and when the RF communications system is in full duplex mode, the receive path is coupled to the antenna and the transmit path. The system further includes the noise cancellation loop for approximating a transmit noise signal from the transmit path and canceling the transmit noise signal from a received signal going to the received path. These and other embodiments, features, aspects and advantages of the present invention will become better understood with regard to the following detailed description, appended claims, and accompanying drawings.